Abstract: "This research investigated the collective interactions between hamstrings extensibility, anterior knee laxity, and hamstrings and quadriceps muscle activation as predictors of anterior knee shear forces during single-leg landings in females. Forty-five healthy, recreationally active females participated in a single data collection session during which hamstrings muscle extensibility and anterior knee laxity were measured, followed by measurement of surface electromyography of the quadriceps and hamstrings and estimation of anterior knee shear forces during single-leg landings. Five single-leg drop landings were conducted from a 30 cm platform positioned 30% of the height of the participant behind the center of a force plate. Electromyographic data were normalized to maximum volitional isometric contractions at 30' of knee flexion, and forces were normalized to body weight. Multiple linear regression analyses were used to evaluate the ability of hamstrings extensibility, anterior knee laxity, and hamstrings and quadriceps pre- and post-landing activation amplitudes to predict initial, rate, and peak anterior knee shear forces during the landings. The primary findings were that hamstrings pre-landing activation negatively predicted anterior knee shear force at initial ground contact and positively predicted the rate of anterior knee shear force development following landing. Furthermore, peak anterior knee shear force following the landings was positively predicted by hamstrings post-landing activation and negatively predicted by hamstrings pre-landing activation. Anterior knee laxity, hamstrings extensibility, and quadriceps pre- and post-landing activations did not significantly add to the prediction of anterior knee shear forces. Hence, it was concluded that hamstrings activation was the primary predictor of anterior knee shear forces during single-leg landings in females. "--Abstract from author supplied metadata.